Contra Costa County Flood Control and Water Conservation District (District) proposes to conduct supplemental geotechnical investigations required to support the geotechnical analyses and detailed design for the proposed Trembath Detention Basin. The geotechnical investigation is planned for fall 2019. The geotechnical investigation includes subsurface exploration that includes four rotary wash borings, one of which will receive a vibrating wire piezometer, seven standard cone penetration tests (CPT) and one seismic CPT. A minimum of three test pits will also be performed within the proposed embankment footprint at locations to be determined after geologic field mapping, test borings, and CPTs have been completed. Test borings will be drilled using track or truck-mounted drilling equipment. CPTs will be advanced using track-mounted CPT equipment. Test pits will be excavated using either a track-mounted excavator or backhoe. Drilling fluids will be circulated in tubs and portable tanks at the drill site. When the fluids need to be disposed of, they will be contained in tanks, drums, or bins and stored in a secure area on site until the end of exploration activities. The tanks/drums/bins then will be transported and disposed of at a landfill. The subsurface exploration program will be used to characterize the subsurface conditions for the planned project elements and to obtain samples for laboratory testing. The boreholes will be backfilled with bentonite, and soil within the upper two feet.